【链表】合并K个排序链表

问题描述

合并 k 个排序链表，返回合并后的排序链表。请分析和描述算法的复杂度。

示例:

输入:
[
  1->4->5,
  1->3->4,
  2->6
]
输出: 1->1->2->3->4->4->5->6

class Solution {
    
    public ListNode mergeKLists(ListNode[] lists) {
    	if(null == lists || lists.length == 0){
    		return null;
    	}

    	int n = lists.length;
        while (n > 1) {
            int k = (n + 1) / 2;
            for (int i = 0; i < n / 2; ++i) {
                lists[i] = merge(lists[i], lists[i + k]);
            }
            n = k;
        }

    	return lists[0];
    }

    private ListNode merge(ListNode node1, ListNode node2){

    	ListNode root = null;
    	ListNode cur = root;
    	while(null != node1 || null != node2){
    		boolean get2 = null == node1 || (null != node2 && node2.val < node1.val);
    		int val = get2 ? node2.val : node1.val;
    		node1 = get2 ? node1 : node1.next;
    		node2 = get2 ? node2.next : node2;

    		if(null == cur){
    			cur = new ListNode(val);
    			root = cur;
    		}else{
    			cur.next = new ListNode(val);
    			cur = cur.next;
    		}
    	}

    	return root;
    }
}